Is Natural Selection Physical?

Is Darwin’s theory of evolution a physical theory? Does anything mental enter into its explanatory apparatus? There are two main components to the theory: mutation and natural selection. It might well seem that these are both entirely physical phenomena, because mutation is just chance genetic change (change in the DNA) and natural selection is the physical impact of the environment on survival and reproduction. We can agree that mutation is a purely physical phenomenon (it results from such things as radiation striking the genome), though we should note that mutations cause mental changes as well as bodily changes; but it is an oversimplification to suppose that natural selection involves no mental elements. If we focus on such factors as climate change, the effects of gravity, lack of food, lightning, and meteors, then we might come to the conclusion that nature selects in virtue of physical facts and events; but these are not the only factors that influence survival and reproduction. Take predators: the bite of a lion certainly affects the survival prospects of its prey, and the bite is physical, but the bite was caused by a desire on the part of the lion—the desire for food. The lion’s psychological state leads it to act in the way it does, shaping the reproductive potential of prey animals. No account of such natural selection would be complete that failed to mention lion psychology. And the same is true of the psychology of the prey: this too affects the chances for survival of an animal hunted by lions. Predation is not just a body-to-body interaction; it is mind-to-mind interaction. The mind plays a causal role in shaping who survives such interactions. An eliminative materialist would deny this, but otherwise it is undeniable. Part of an animal’s environment is the psychological environment (including its own) not just the physical environment; so natural selection includes psychological factors. An animal is adapted to its physical environment and to its psychological environment. This is true even for plants: their selection is governed in part by the behavior of herbivorous animals, which typically results from the psychological state of such animals—does the animal like to eat this type of grass?

In the case of social animals this kind of selection by psychological environment is even more pronounced. Competition is a powerful driver of evolution, but competition involves the psychology of the competitors: aggressive conspecifics will decisively affect an animal’s opportunities for reproduction, and aggression involves emotional states. This is why many animals need a theory of mind to navigate their social life: you need to understand others’ psychology in order to survive and mate. If the psychological environment were to change, what was once adaptive could become less so. What is called artificial selection illustrates the point beautifully: this kind of selection is done by conscious agents with intentions, for example dog breeders. Here selection is governed by the aesthetic tastes of the breeders or by marker expectations; no account of this process could be correct that failed to recognize this selective force. Nor is it really outside the domain of natural selection, since there is nothing unnatural about the human desire to shape other species to their own ends (bees “artificially” select flower types too). If a species did this in order to provide a food source better suited to its needs, that would be perfectly within the natural domain—what Darwin called artificial selection is just another kind of natural selection (it isn’t super-natural selection). Once organisms get minds natural selection is influenced by these minds, so it is not always a purely physical matter. Of course, if we are materialists we will suppose that minds are really material, so that all natural selection comes down to physical selection; but that does not gainsay the point that mental phenomena are involved in the selection process—just not irreducibly mental phenomena. And the Darwinian theory itself is not committed to any such materialism: it simply speaks of whatever factors there are that can shape natural selection, physical or mental. In some possible worlds no doubt the only form of natural selection is psychologically driven—organisms are never selectively acted upon by purely physical phenomena.

Sexual selection is another case in point. Again, this should not be opposed to natural selection, since it is just a type of natural selection—the type in which mates select each other by considerations of fitness and attractiveness, as with the peacock’s tail. Here the selective force is an estimate of the fitness of the potential mate—that is, what the selector thinks is attractive in a mate. It is a matter of psychological response, and this response will determine whether the potential mate becomes an actual mate. In some species sexual selection is the main determinant of reproductive success—much more so that lightning strikes or falls from great heights or poisonous berries. If you are selected for your looks, then it is the aesthetic tastes of the selector that determine whether your genes get passed on. Sexual selection is psychologically driven and a powerful selective force. Of course, such psychological causation is physically mediated by actual behavior, in this case and in the others I have mentioned; but that is not to say that the selection is purely physical, since it stems from psychological factors. You would not want to say that selecting a wife or husband is a purely physical matter just because the outward acts that are involved are physical.

So Darwin’s theory is not in any way an elimination of the mental from the process of evolution: the mechanism of natural selection operates over psychological facts as much as physical facts. Nature selects both physically and mentally. It is not that Darwin’s theory replaces the mental with the physical in its account of evolution, by (say) removing God’s intentions from the picture. It is not a scientific theory that would gladden the heart of a metaphysical materialist. It is not a physical theory in the way chemistry is. It uses the mental; it doesn’t eschew it. Nor does it take a stand on the nature of the mental: it is neutral with respect to the metaphysics of mind.[1] This is simply because nature includes more than physical nature. Mind accordingly played a role in the origin of species.[2]

[1] You cannot read off from Darwin’s theory which theory of the nature of mind is correct: materialism, functionalism, anomalous monism, supervenience, panpsychism, computationalism, dualism, etc. This is not to say that theories don’t differ in how easy it is to explain the evolution of mind.

[2] I doubt that anyone would disagree with this once it is spelled out. Nevertheless, there seems to be a vague feeling out there that Darwin’s theory has materialist tendencies, or even that it claims to account for all of evolution without mentioning anything mental. Clearly minds play a role in determining what traits get selected and passed on, according to the theory.